1. Field of the Invention
The present invention relates to copolymers of ethylene which are suitable for forming foams.
2. Discussion of the Prior Art
Foamed plastics are used in many fields of industry. They are especially used as packaging material and for heat and sound insulation. Various thermoplastic materials are suitable for preparing them. Examples include polyurethanes, polystyrene, polyethylene, rigid and plasticized polyvinyl chloride. In a known process, a chemical blowing agent which is decomposed in the melt of the thermoplastic materials is used. During the heat treatment, there is an evolution of gas which causes foaming of the composition and formation of pores in the plastic. According to another known process, an inert gas is dissolved in the plastic material under a very high pressure. Thereafter the pressure is released, and a foam is formed by expansion of the gas.
These so-called blowing agent processes have the disadvantage that foaming must generally be effected in two steps. The first step is operated with closed molds placed in presses, and the second stage involves post-foaming under atmospheric pressure and at a temperature lying slightly above the softening point.
Japanese Patent Application No. 71/36 474 (Chemical Abstracts, Vol. 77, 1972, Abstract No. 89 428 f) describes the preparation of foams from thermoplastics without the addition of blowing agents. In this process, the starting materials are copolymers which on heating cause foaming by cleavage of gaseous decomposition products. Copolymers of ethylene with a comonomer such as vinyl acetate, vinyl butyrate, methyl acrylate or methacrylic acid are used as plastics which are heated in the absence of oxygen, i.e., under reduced pressure or in an inert gas atmosphere to temperatures at which cleavage of the polymer chains does not yet occur.
This process has the disadvantage that foams are obtained which are only slightly cross-linked, the pore structure of which is only slightly evident and can hardly be influenced by variation of the composition of the starting materials and the preparation of which requires reaction times of about 2 hours.
Therefore, the problem had to be solved of providing thermoplastics for the preparation of foams which, on foaming, avoid the deficiencies of the prior art processes and permit the preparation of the foam in commercially feasible and simple means.